Study claims lunar caverns could hold city-size colonies

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Lunar rilles similar to those explored by Apollo 15 in 1971 indicate the existence of giant lava tubes on the Moon (Image: NASA)

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We tend to think of future lunar colonies as being cramped futuristic domes or subsurface rabbit warrens, but what about underground caverns big enough to hang glide in? According to a study by a Purdue University team, that may not be so daft as it calculates that the Moon may contain lava tubes large enough to house entire cities.

The Moon may have a stark, silent majesty about it, but as a potential home it makes the Gobi desert look like paradise. Its atmosphere is only an academic argument away from being hard vacuum, it's surface is blasted by solar and cosmic radiation, its temperature during its month-long day ranges between far above boiling to deep below freezing, and it's constantly peppered with meteorites.

With that sort of environment, it's small wonder that scientists and engineers favor setting up shop underground rather than on the lunar surface. One option would be to excavate, and another might be to seek out pits on the Moon, but if you really want to build serious colonies, Purdue suggests lava tubes.

Lava tubes are formed during volcanic eruptions by flowing lava, either as troughs that crust over or tunnels deep beneath the surface. As the eruption ends, the lava drains away, leaving behind a tube-shaped cavern. On Earth, lava tubes can be 14 to 15 m (46 to 49 ft) wide, and up to 50 km (31 mi) long. Scientists believe that not only could such tubes exist on the Moon, but they'd be much larger.

The best direct evidence of lunar lava tubes are the long, sinuous trenches or valleys (known as rilles) measuring up to 10 km (6 mi) wide that are found on the lunar surface. They most famous of these is Hadley Rille, which was explored by the Apollo 15 expedition in 1971. These rilles, which resemble dried river beds, are believed by lunar geologists to be collapsed lava tubes and their width gives some indication of their potential size.

The question is, could lava tubes as wide or larger than the rilles seen on the lunar surface exist? The Moon has only a sixth of the Earth’s gravity and the forces of erosion that weather down the Earth's surface don't exist, so it's possible. Previous studies in the late '60s and early '70s indicated that lunar lava tubes exist 40 m (130 ft) beneath the surface and are 300 m (980 ft) wide.

Cross section of a theoretical lunar lava tube with the city of Philadelphia for scale (Image: Purdue University)

The latest Purdue study used current information about lunar rocks, geology, gravity, and environment and applied terrestrial principles of civil engineering. The researchers calculated that, given the potential width of the tubes, roof thickness, and the stress state of the cooled lava, the caverns could, barring major earthquake or meteorite impacts, be surprisingly large.

"We found that if lunar lava tubes existed with a strong arched shape like those on Earth, they would be stable at sizes up to 5,000 meters, or several miles wide, on the Moon," says Purdue graduate student David Blair. "This wouldn't be possible on Earth, but gravity is much lower on the Moon and lunar rock doesn't have to withstand the same weathering and erosion. In theory, huge lava tubes – big enough to easily house a city – could be structurally sound on the Moon."

The upshot of this is that it's possible that the Moon could have tunnels wide enough and long enough to easily hold Manhattan Island. Such tunnels would, of course be airless, but they would be well protected, with a stable temperature of -20° C (-4° F), so with proper pressurization and moderate heating they could be made habitable. Another advantage is that since the lava tubes would be located by the highlands, they'd be well located for mining operations as well as landing areas in the flat maria.

How good they would be for hang gliding remains to be seen.

The team’s results were presented at the Lunar and Planetary Science Conference.

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